Fertilizers have been used for many years to supplement nutrients in growing media.
In recent years the fertilizer arts have focused on techniques to deliver controlled amounts of plant nutrients to soil or other growing media. The goals of controlled release of fertilizer are (1) to ensure that growing plants are not adversely deprived of nutrients, and (2) to ensure that an over supply of nutrients is avoided. An over supply of nutrients can result in toxicity to plants or losses from leaching. Controlled release of fertilizers provides an improvement in fertilizer use efficiency and can reduce the rate and the frequency of nutrient application.
A controlled release product that has a low coating weight or thin coating is desirable to save manufacturing costs, since less coating is required. In order to have a good slow release profile however the coating must be substantially uniform. This is difficult to achieve in a thin coating, because during conventional coating processes the coating is often damaged or its integrity impaired.
It has surprisingly been found that if the coating process is separated into two steps, an application step and a stabilization step, where different operating parameters (i.e., mechanical handling of substrate and coated substrate) are used for each step, a controlled release product can be made having a low coating weight and a good slow release profile.
A single operating parameter between the application and stabilization steps of latex on a fertilizer is described in U.S. Pat. Nos. 5,089,041 and 5,186,732, where a single fluidized bed is used for both steps. A similar process is described in U.S. Pat. Nos. 5,188,654, 5,256,181 and 5,435,821 for the application of polymer to a fertilizer.
In a series of patents by Moore (U.S. Pat. Nos. 4,711,659, 4,804,403 and 4,969,947) a polymer topcoat is created by applying to a fertilizer a first layer, which chemically bonds to the fertilizer, and a second layer, which chemically bonds to the first layer. While a rapidly rotating drum is described in the Examples for the application step, a change of operating parameter is not described for the stabilization step.
A process for coating a fertilizer with polymer is also described in U.S. Pat. No. 6,358,295. The tumbling motion described for the application step is apparently maintained for the stabilization step.
It has also been suggested that where a polymer coating is applied, fertilizer particles are to be kept in low shear, low impact, motion relative to each other during the coating process (i.e., gentle mixing): U.S. Pat. Nos. 5,538,531, 5,851,261, 5,698,002, 6,358,296, 6,364,925 and 6,503,288.
Composite coatings comprising a polymer and an additive/filler have also been described in U.S. Pat. No. 6,663,686, US 2004/0020254 and US 2004/0016276, but only a single operating parameter is described in the patent and publications.
In addition to polymer coatings, other fertilizer coatings have also been used, such as a cement only or cement/elastomer coating (U.S. Pat. No. 4,023,955); an amine or amine blend with a microcrystalline wax, paraffin or synthetic wax (U.S. Pat. No. 6,475,259); an alkylamine mineral oil combination (U.S. Pat. Nos. 4,150,965 and 4,220,463); and a urea-lignosulfonate binder (U.S. Pat. No. 5,238,480). In these cases as well, different operating parameters have not been described for the application and stabilization steps.
A nitrogen based coating material for calcium has also been described in U.S. Pat. Nos. 5,997,601 and 5,917,110. A change in operating parameters between the application and stabilization steps is not described in these patents. In these patents the problem of agglomeration of coated particles is avoided by application of a conditioning agent.
The patent literature also describes coating a fertilizer with sulphur alone or in combination with a polymer or another coating material. There are inherent problems in coating a fertilizer with sulphur. In sulphur-only coated fertilizers, specific processing parameters must be used to avoid undesirable forms of sulfur, which crack, resulting in loss of controlled release properties (see for instance U.S. Pat. Nos. 3,903,333 or 5,405,426). Pre-treatment of fertilizer may be required to avoid these problems: U.S. Pat. Nos. 3,903,333 and 5,563,782. Temperature control throughout the coating process is also a key factor (U.S. Pat. No. 3,903,333).
Controlled release properties of sulfur coated fertilizers may also be improved by utilizing a polymer topcoat as in the case of U.S. Pat. Nos. 5,219,465, 5,405,426 and 5,466,274. Wax based sealants have also been employed (U.S. Pat. Nos. 4,042,366, 5,300,135, 5,466,274, 5,478,375 and 5,984,994). Attempts to improve the controlled release properties of sulfur-coated fertilizers have also been made by mechanically interweaving the sulfur and fertilizer (U.S. Pat. No. 4,857,098). Polymer-sulfur-polymer coated fertilizers have also been employed (U.S. Pat. No. 6,338,746).